Cannabis and IQ

By George Gifford

A recent paper by Ole Rogenberg has challenged previous findings that adolescent cannabis use may lower IQ. The research in question by Meier et al. (2012) found that those who started smoking cannabis weekly before age 18, and carried on regularly use into adulthood, showed an average 8 point drop in IQ. This study involved a sample of 1,037, who were followed from birth to age 38, with five intermittent interviews ascertaining drug use, and IQ tests at ages 13 and 38.

New criticism of this work comes in the form of a simulation statistical model suggesting that the results in Meier et al. (2012) could be confounded by the variable of poverty, those with lower socioeconomic status being more likely to smoke cannabis from a younger age and do less well in school. In reply to this researchers from Meier et al. (2012) have stated that when they restrict their data to only including those from middle-class homes their results are the same, suggesting that Rogenberg’s claims are not represented in their data. Despite this, critics such as Rogenberg still maintain that Meier et al. (2012) does not account for potentially confounding factors and their results do not suggest the neurotoxicity of cannabis as claimed. Put simply, Meier et al. (2012) think their results show a biological effect of cannabis on young brains and Rogenberg (2012) does not.

This controversy here is somewhat reminiscent of the controversy surrounding studies which have found differences in IQ between groups with different racial origin. Most importantly, whilst some researchers have previously chosen to interpret results on a biological basis, IQ has often been shown to be affected by some very un-biological and quite subtle factors. For example, one study by Day and Anthony (2006) found that black Americans only underperformed white Americans in IQ tests when they were told the tests were measures of their performance, rather than a set of puzzles the researchers wanted their opinions on. This supports the ‘stereotype threat’ explanation of racial IQ differences, which suggests that anxiety about confirming negative stereotypes in such situations can affect performance.

We could perhaps apply ‘stereotype threat’ to cannabis users, most of whom we could presume would be well aware of negative stereotype of the underachieving stoner. This could relate to how cannabis using participants engaged with education, from their happiness and motivation in school, to how they were treated by parents and teachers. This means that regardless of how Meier et al (2012) controlled for education by accounting for years spent at school and university, education may still be the mechanism in which cannabis affected the cohort’s IQ scores.

Such alternative explanations for the differences in IQ scores do not prove that smoking cannabis does not have an effect on developing brains. It does however highlight that we have an incomplete understanding as to how this would work, and no definite answer as to why cannabis affected later IQ in Meier et al (2012).

This point is thankfully not lost on researchers, but may not be apparent to many of those on either side of the cannabis debate.